Nephilim. Now there has been two different myths floating around mythology concerning these guys. Th first is that they are, in and of themeslves, fallen angels. The other is that they were the offspring of fallen angels, namely the Grigori/Watchers. The first version is a little more ill suited as it was more based on the Akadian and Sumerian account of their "alien" gods from the stars (heavens), this has more to do with non chrisitan or the standard Abrahamic religions, wich is what I use for fantasy. So I went with the "children" theme. As you see here the are half human half Grigori/Watchers. In some myths they were giants, and some are, but those are called giborum, more on them later.